1. Field of the Invention
The present invention relates to a microporous sheet, particularly a substrate for artificial leather; and to a process for production thereof. More specifically, the present invention relates to a microporous sheet which is obtained by impregnating a non-woven fabric with an elastic polymer and which is capable of controlling properties such as softness, abrasion resistance, tensile strength, tear strength and the like easily and appropriately to a desired extent depending on the purpose and application; and to a process for production of the microporous sheet. The microporous sheet of the present invention can be favorably used as a substrate for artificial leather.
2. Prior Art
In general, that substrates for artificial leather which can be made into a full-grain type artificial leather by coating a high polymer on a surface of the substrate or can be made into a suede type artificial leather or a nubuck type artificial leather by grinding the surface of the substrate, are produced by impregnating a woven fabric, a knit fabric or a non-woven fabric as a base fabric with a high polymer, particularly by impregnating a non-woven fabric with an elastic polymer (e.g. a polyurethane) in view of the strength and durability of the resulting substrate. In production of a microporous sheet suitable as such a substrate for artificial leather, however, impregnation of a fibrous base (e.g. a non-woven fabric) with a solution of an elastic polymer dissolved in an organic polar solvent such as dimethylformamide is followed by coagulation of the polymer of the impregnated solution in water, which results in adhesion of the elastic polymer to the fiber of the fabric and gives a natural leather substitute which is difficult to elongate and has abrasion resistance but is hard, and consequently has limited applications. Therefore, measures have been taken in the industry to prevent the adhesion of the elastic polymer to the fiber the fabric. For example, Japanese Kokai (Laid-Open) Patent Application No. 9839/1972 corresponding to U.S. Pat. No. 3,811,923 discloses a method which comprises applying, on the fiber of the fabric, an agent such as a silicone and the like, having a releasing effect on an elastic polymer, prior to the impregnation of the fiber with an elastic polymer. In this method, when the solvent used in the impregnation solution containing the elastic polymer is water, the adhesion of the elastic polymer to the fiber is prevented so that the fiber can have high freedom and a soft microporous sheet suitable as a substrate for artificial leather can be obtained. But, when the solvent is an organic polar solvent (e.g. dimethylformamide), the effect of the release agent is small and it is impossible to obtain a soft microporous sheet. Furthermore, when the fiber is covered with an agent having a releasing effect such as a silicone, a soft microporous sheet can be obtained because the elastic polymer does not adhere to the fiber, as mentioned above, but disadvantages are also increased that the resulting fabric is easy to elongate due to reduction in friction coefficient between fibers, has reduced abrasion resistance, and the like.
The method for production of a microporous sheet so as to allow no adhesion between polymeric polymer and fiber, includes also a method as described in, for example, Japanese Patent Publication No. 31955/1973, which comprises applying, on a surface of a fiber, a polymer (e.g. a polyvinyl alcohol) which is water-soluble but insoluble in dimethylformamide, impregnating the resulting fiber with a solution of a polyurethane dissolved in dimethylformamide, coagulating the polyurethane of the impregnated solution in water, and removing the polyvinyl alcohol by water washing. In this method, the adhesion between the polyurethane and the fiber can be prevented so that the fiber can have high freedom and a soft substrate for artificial leather can be obtained. In this case as well, however, while softness can be obtained, disadvantages appear that it is easy to elongate, has reduced abrasion resistance, and the like. That is, when the polyurethane and the fiber are completely bonded to each other, the fabric can have advantages that it has excellent abrasion resistance, is difficult to elongate, and the like, but has disadvantages that it is hard, has reduced tear strength, and the like. Conversely, when the polyurethane and the fiber are not completely bonded to each other, the fabric can have softness but has reduced abrasion resistance, and becomes easy to elongate.
In recent years, artificial leathers have gained wide acceptance in applications such as shoes, balls, furnitures, garments, gloves and other sundry goods. The property requirements for artificial leather vary depending upon the application and the kind of the fabrication. In order to produce artificial leathers well suited for wide applications or fabrication methods, the techniques used heretofore have a limitation.
Hence, the present inventors made extensive studies in order to provide a microporous sheet suitable as a substrate for artificial leather, in which sheet the proportions and densities of (a) portions where a fiber of the non-woven fabric and an elastic polymer are bonded (or adhered) to each other and (b) portions where a fiber and an elastic polymer are not bonded (or adhered) to each other can be easily controlled so as to meet the application of the microporous sheet and the fabrication; and to provide a process for production of the microporous sheet.
As a result, the present inventors have found that when a specific surfactant is dissolved in a solution of an elastic polymer to impregnate a non-woven fabric with the resulting solution and the polymer of the impregnated solution is coagulated in water, the fiber of the fabric and the elastic polymer are bonded or not bonded to each other depending upon the kind of the polymer impregnated into the fiber.
Thus, the present inventors have found that by forming a non-woven fabric from at least two kinds of fibers and further by changing the proportions of the different fibers in the non-woven fabric and impregnating the non-woven fabric with an elastic polymer solution containing a specific surfactant, there can be obtained a microporous sheet which has (a) portions where the fiber and the elastic polymer are bonded to each other and (b) portions where the fiber and the elastic polymer are not bonded to each other, in desirably controlled proportions and which has a desirably controlled balance in softness, abrasion resistance and strength. The present invention has been completed based on the above finding.
According to the present invention there is provided a microporous sheet obtained by impregnating a non-woven fabric with an elastic polymer solution and then coagulating the polymer, wherein (1) said non-woven fabric is a blend of (a) an aromatic polyester fiber (fiber A) and (b) a polyolefin or polyamide fiber (fiber B), and (2) the microporous sheet is (i) scattered with the portions where the fiber A is surrounded by the elastic polymer in a bonded state and the portions where the fiber B is surrounded by the elastic polymer in a not-bonded state, and has (ii) a softness of 0.5 to 6.0 and (iii) an abrasion resistance of 1,500 to 8,000.
According to the present invention there is further provided a process for producing a microporous sheet by impregnating a non-woven fabric with a solution of an elastic polymer dissolved in an organic polar solvent and then coagulating the polymer of the impregnated solution in a coagulation bath composed mainly of water, wherein the non-woven fabric is a blend of a polyester fiber (fiber A) and a polyolefin or nylon fiber (fiber B) and the organic polar solution is a solution containing 0.1 to 10 parts by weight, per 100 parts by weight (as solid content) of the elastic polymer, of a water-dispersible or water-soluble surfactant having a silicone segment as a hydrophobic group.